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Strap Testing

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DerekSmith:
An interesting review of strap performance, flat, twisted and knotted.

https://www.youtube.com/watch?v=ifyJjQXOttE

Putting a single twist into my tiedown straps seems to be an excellent amendment, especially as I never get anywhere near MBS, but flogging and vibration can do serious damage to the strap and the load.

Derek

agent_smith:
Thanks Derek - I saw this video earlier this week.
Some interesting observations.

As with all 'enthusiast/home brew' testing, there isn't a huge sample size with each round of testing (eg 3 Sigma).
However, the results seem to follow a consistant pattern.
And I would also comment that these types of tests are always welcome!

Key points:
[ ] Breaks normally occurred at the ratchet end of the test rig - specifically at the rotating spindle - and always with evidence of heat build up causing melting - he did not advance a theory as to why the breaks always occurred at this point (with the exception of the knotted test)
[ ] The web strap suffered compression at the jack head (but no breaks at this location)
[ ] Weakest configuration was a knotted web strap - as we would expect
[ ] A few twists causes no harm - ie no significant reduction to MBS yield
[ ] wet webbing suffers some strength loss (as we would expect) - approx 10% reduction in MBS yield

...

With failure always propagating from the spindle of the ratchet, someone should be able to advance a theory to explain this...
# Likely cause could be due to tight radius that the webbing belt has to bend around - and with multiple turns around the spindle - the webbing is progressively compressed/squeezed. There will be both compression and stretching over the cross-section thickness of the webbing material - which will cause frictional heating. All of this results in a stress concentration at the spindle - which eventually leads to rupture.

DerekSmith:
Hi Mark,

I concur with your rationale for the consistent failure at the entry to the ratchet spindle, it being the sharpest radius in the whole system.

I am interested in you suggestion that we would expect a 10% reduction in MBS for a wet braid.  A counter argument to this would be that water would prevent the braid from exceeding 100C and therefore prevent melting and subsequent weakening.  Taking this single result at face value, do you have a perspective as to why wetting the braid should weaken it?

Of course, with synthetic fibres it is possible that the braid did not melt, but suffered from pressure flow deformation and the heating happened as the work stored in the elongated fibres was released at rupture, leaving the braid momentarily hot.  In my own trials, although at a much smaller scale, I was never able to show heating during the loading phase.

Derek

Dan_Lehman:
I've only glanced --sans sound-- at the video.
WOW, some kind of workshop!!

> ... 10% loss when wet

I presume that this presumes --is it anywhere stated?--
that the material is nylon, which weakens when wet
(and in towing, apparently has internal heating/friction
issues, too (!)).

Even at a some-guessed 50% weakening, note that the
WLL is 1/3, so there'd yet be margin.  But I don't think
that that much weakening was shown, was it?

Thanks!
 :)

SS369:
Here?s a link to read. http://jur.byu.edu/?p=21529
Basically if reads that wet nylon tubular webbing does not see a significant strength loss.

Regarding the tie down straps in the video, imo it is a slight angular difference at the ratchet that allows one side of the strap to exceed the maximum strength where it initiates failure. It looked to me as though all the breaks occurred away from the spindle. Unless the strap was moved for clarity (?)

Owning a few of the same straps, they are nylon.

SS

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